exp run: Use hydra for parsing --set-param.

[daavoo]

• Overriding a config value : foo.bar=value
• Appending a config value : +foo.bar=value
• Appending or overriding a config value : ++foo.bar=value
• Removing a config value : ~foo.bar, ~foo.bar=value

See https://hydra.cc/docs/advanced/override_grammar/basic/#modifying-the-config-object

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💣 Breaking changes:

• To modify a list, foo[0]=bar must now be passed as foo.0=bar.

• Converting a dictionary to a list is not supported by omegaconf.

None of the two were documented.

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Closes #4883
Closes #5868
Closes #6129

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

[daavoo]

To modify a list, foo[0]=bar must now be passed as foo.0=bar.

[daavoo]

Converting a dictionary to a list is not supported by omegaconf.

[daavoo]

All these 4 functions had to be added because we try to preserve comments while modifying the params file.

I don't know if this comment-preserving logic was a user request or how relevant it actually is.
For example, YAML spec says:

Comments are a presentation detail and must not have any effect on the serialization tree or representation graph. 
In particular, comments are not associated with a particular node.

If we discard the idea of trying to preserve comments we could remove quite a lot of code

[dberenbaum]

Python support is still pretty buggy but if anything is probably better than before (cc @skshetry)